1. Field of the Invention
The present invention relates to an optical card.
2. Related Background Art
In recent years, with the progress of information-oriented society, as the means for handling numerous and various kinds of informations with good efficiency, there have been proposed a large number of information recording carriers and devices for recording and reproduction of informations which perform recording or reproduction of optical informations such as optical disc, optical card, optical tape, etc. As the above information recording carrier, there is one in which the information converted into quantized values is detected by conversion into the change in intensity of reflected light accompanied with change in reflectance, change in surface shape due to presence of a pit.
As the recording carrier depending on the change in optical reflectance, there has been proposed the so called heat mode recording material in which the recording layer is irradiated with an energy beam such as laser beam in shape of a spot, whereby a part of the recording layer is changed in state to effect recording. These recording materials are the so called DRAW (direct read after write) media, which can "direct read after write" without requiring development processing, etc. after writing of the information, enabling high density recording and also additional writing.
In devices for recording and reproduction of optical informations, there is the system in which grooves are previously formed at certain intervals for track servo of writing and reading. According to this system, since the grooves play a role as the guide for reading, the track control precision of laser beam is improved, whereby higher speed access than in the system by use of grooveless substrate of the prior art becomes possible.
For formation of track grooves on a substrate, when the substrate is a thermoplastic resin, there have been known in the art the method in which a stamper pattern is heat transferred according to the method such as injection molding or hot press molding at a temperature higher than the melting point, or the method in which a photocurable resin is coated on the substrate, a stamper pattern is closely adhered thereon and said photocurable resin composition is cured by irradiation of UV-ray, etc., thereby phototransferring the stamper pattern.
Next, description is made by referring to an example of optical as the optical information recording carrier. FIG. 1 is a schematic sectional view of the optical card of the prior art. In FIG. 1, 1 is a substrate, 2 a track groove portion, 3 an optical recording layer, 4 an adhesive layer and 5 a protective substrate.
In FIG. 1, recording and reproduction of information are performed by optical writing and reading through the substrate 1 and the track groove portion 2. In the optical card of the prior art, in the case of the heat transfer method as described above, the track groove portion 2 is integrally made with the substrate 1 to be made of the same material, while in the case of the optical transfer method, the track groove portion 2 is not made of the same material as in the substrate but under the adhered state. In either case, the track groove corresponding to the guide groove for light is previously formed so as to perform tracking through the phase difference of laser beam by utilizing fine unevenness. Also, an optical recording layer 3 is formed on the track groove portion 2 and a protective substrate 5 is plastered through an adhesive layer 4. Thus, the optical card can be easily prepared.
In the method of tracking for the above optical card, laser beam is irradiated in a spot so as to stride across a track groove, whereby tracking is effected by utilizing the phase difference of reflected light through unevenness of the track groove portion.
Also, on the substrate is provided a recording layer subjected to patterning without provision of a track groove which will give rise to phase difference, thereby effecting tracking by utilizing the dose ratio of reflected lights at the recording layer portion and the portion where no recording layer exists relative to the incident light of the laser beam, namely amplitude.
However, in the former case, tracking servo control is performed by effecting tracking so as to create phase difference in reflected lights in the depth of track groove, the groove width and the track groove, but due to variance in dimensions of the track groove width or unevenness of the track groove, fluctuation of the light source used such as laser beam, etc. and the dose distribution of the laser beam (depending on the spot diameter), etc., there is the problem that the tracking servo control cannot be completely performed only through the phase difference in reflected lights relative to the spot-shaped incident light.
On the other hand, in the latter case, the preparation method is performed by printing, etc. and therefore no fine working can be obtained with good productivity.
Particularly, in an optical card medium, as different from optical disc, etc., recording and reproduction of information are performed by reciprocal movement of optical card medium and parallel movement of laser recording head. Accordingly, in an optical card medium, as compared with the recording and reproduction of information by circular movement as in optical disc, etc., errors for determination of correct recording and reproduction positions during recording and reproduction of information tend to be increased.
For reducing such errors, in an optical card medium, it becomes necessary to obtain a tracking signal of high precision and high quality.